What Does Your Building Project Really Cost?
Plan a budget for building, renovation and energy work across London and the UK. Use the free calculator to explore indicative costs, then compare written contractor quotes for your project.
Grant treatment: Calculator totals show the budget before any grants. Grant availability and eligibility must be confirmed for your property by an eligible installer; no grant is deducted automatically.
Start here: how to use FairBuildCost
Planning building work? Enter your postcode below, choose the work and specification, then review your selection and enter your email. You will receive an indicative estimate where a matching scope is available, or a review acknowledgement listing the details needed before pricing. For extensions and lofts, enter a whole-number area from 1 to 500 m². For multiple work items, provide each area separately in the project notes; a scope review is required before a combined price can be prepared.
Improving energy performance? You can select energy work manually. Your EPC recommendations are not yet imported, and this calculator does not predict a new EPC rating. Read the energy cost guide and confirm the measures needed for your property.
Already have a quote? Use Quote Review to request an indicative review. Builders, installers and property professionals can use the calculator too, or contact us about their requirements. An estimate is a budgeting aid, not a contractor quote, survey or EPC certificate.
Get a Free Building Estimate by Email
Enter your postcode and select the work you’re planning. Review your selected work and receive an email. Where the scope needs confirmation, the email lists the details needed before pricing. This is not a contractor quote.
Help us improve: Share your feedback — tell us what worked, what was missing or what you would like us to add.
Indicative estimates only. Always obtain written quotes from qualified contractors.
Why Building Costs Are So Hard to Budget
Budgets can change when the measured scope, existing structure or chosen finishes differ from initial assumptions.
The reason isn’t dishonest contractors. It’s that building costs are genuinely complex — and most online cost guides show minimum figures, not realistic ones.
A published starting price may cover a different size, specification or scope from your project.
Compare the included works, quantities, location, VAT basis and exclusions before comparing prices.
FairBuildCost uses stored rates to produce an indicative budget. VAT assumptions and available scope notes are shown in your email. Confirm contingency, professional fees, compliance costs and exclusions in a written contractor quote.
Choose Your Intended Specification
Choose your intended specification. A preference does not confirm the included work; some selections require a scope review before they can be priced.
| Level | Who It’s For |
|---|---|
| Budget | Rental properties, fast turnaround, cost-first decisions. Entry-level materials, local contractors. Agree a contingency allowance separately. |
| Standard | Most homeowners. Mid-market materials, experienced contractors, solid finish. Agree a contingency allowance separately. |
| Premium | Owner-occupiers wanting quality finish. High-spec materials, specialist contractors. Agree a contingency allowance separately. |
| Luxury | High-value London properties. Architect-led, bespoke, fully project managed. Agree a contingency allowance separately. |
The scope varies by work type and specification. Do not assume all labour, materials, professional fees, compliance costs or contingency are included unless expressly stated.
Cost Guides for Every Type of Building Work
Extensions — Single storey · Double storey · Side return · Wrap-around · Garage conversion
Loft Conversions — Velux · Dormer · Hip to Gable · Mansard
Renovation & Refurbishment — Full house · Kitchen · Bathroom · En-suite · Structural alterations
Basement & New Build — Basement conversion · Basement excavation
Energy Installations (DEH & P LTD) — Heat pump · Solar PV · Battery storage · Air conditioning · MVHR · EV charger
Maintenance & Compliance — Roof replacement · Damp proofing · External wall insulation · Windows · Rewire
London Building Costs by Zone
Building work in London costs significantly more than the UK average — and varies substantially between zones.
| Zone | Areas | Cost vs UK Average |
|---|---|---|
| Zone 1–2 | Mayfair, Chelsea, Kensington, Westminster, City | +50–65% |
| Zone 3–4 | Clapham, Fulham, Hackney, Islington, Battersea | +35–45% |
| Zone 5–6 | Croydon, Barnet, Bromley, Harrow, Enfield | +15–25% |
| South East | Surrey, Kent, Hertfordshire, Berkshire | +10–15% |
| UK Average | Midlands, North, South West, Scotland, Wales | baseline |
How FairBuildCost Works
Step 1 — Select your work type. Choose from the full list of building, renovation and energy work types.
Step 2 — Enter your postcode. We apply the correct London zone or regional multiplier automatically.
Step 3 — Review your selection and enter your email. Choose a specification and receive an estimate or a request for the details needed to review the scope.
Step 4 — Understand what’s included. Your email shows the selected work, estimated costs and available scope notes. Confirm any unspecified inclusions, exclusions and permission requirements with a qualified professional.
How Building Work Affects Your Property Value
Understanding what you’ll spend is only half the picture. Understanding what you’ll gain is equally important.
The figures below are illustrative planning scenarios, not an address-specific valuation or a verified prediction of resale value. The current calculator does not connect to EPC or Land Registry records. Obtain a local professional valuation before relying on a potential increase in property value.
| Work Type | Illustrative scenario (London Z3–4), not a valuation |
|---|---|
| Single storey extension | +£60,000–£90,000 |
| Dormer loft conversion | +£75,000–£112,500 |
| Full house renovation | +£75,000–£135,000 |
| Air source heat pump | +£7,500–£22,500 |
| Solar PV + battery | +£7,500–£15,000 |
Energy Installations — Featured Specialist: DEH & P LTD
For heat pump, solar PV, air conditioning and battery storage installations, FairBuildCost features DEH & P LTD as its recommended energy specialist.
DEH & P LTD — Energy installation services covering Surrey, London and the South East. Heat pumps · Solar PV · Air conditioning · Battery storage
Key facts:
- Heat pump installations qualify for 0% VAT
- BUS Grant: £7,500 available for air source and ground source heat pumps
- For eligible renewable installations, check the installer’s current MCS certification and the technologies covered. MCS does not certify this cost calculator.
- DEH & P manages your BUS grant application
Get a heat pump or solar quote from DEH & P → dehp.uk
Why Trust These Figures?
How estimates are produced. The current calculator uses stored budget rates, postcode zones and your selected specification. EPC and Land Registry integration are planned features; these registers do not currently determine your estimate.
Check the review date. Check the review date shown for the rate or guide. Confirm current local costs before committing to work.
VAT always stated. Every figure on this platform states clearly whether VAT is included or excluded. Estimates show the VAT assumption stored for the selected work. Eligibility for a reduced or zero rate must be confirmed for the installation.
Price sources and limitations. Our initial comparison uses published guidance from Beams on extensions and Energy Saving Trust on heat pumps. These are external benchmarks, not verification of every stored rate. Prices and inclusions remain subject to project-specific confirmation.
Who operates this site. FairBuildCost is a standalone building-cost information platform operated by DEH & P LTD. You can use its estimates with any contractor. DEH & P is featured for energy work, and may benefit commercially if you choose its services.
All figures published on FairBuildCost.co.uk are indicative estimates for budgeting purposes only. Actual costs depend on site conditions, structural survey results, planning requirements, specification and contractor pricing. FairBuildCost.co.uk does not provide quotes, does not guarantee costs and is not a contractor. Always obtain at least three written quotes from qualified, insured contractors before committing to any building project. Costs last reviewed: June 2026. Next review: September 2026.